Sixteen people were injured when a Greyhound bus with 42 people on board overturned on the N3 freeway at Montrose on Tuesday morning, KwaZulu-Natal paramedics said.

The bus had apparently broken down on the roadside, about 20km outside Harrismith.

People were working on the vehicle when it started rolling down a slope while passengers were still aboard, said Netcare 911 spokesperson Chris Botha.

"It overturned and came to rest on its roof."

Paramedics initially reported it was a TransLux bus but later verified it was a Greyhound bus.

Botha said one person sustained serious injuries while 15 others had minor bruises.